Anaerobic microorganisms can produce ethanol from CO through fermentation of gaseous substrates. Fermentations using anaerobic microorganisms from the genus Clostridium produce ethanol and other useful products. For example, U.S. Pat. No. 5,173,429 describes Clostridium ljungdahlii ATCC No. 49587, an anaerobic microorganism that produces ethanol and acetate from synthesis gas. U.S. Pat. No. 5,807,722 describes a method and apparatus for converting waste gases into organic acids and alcohols using Clostridium ljungdahlii ATCC No. 55380. U.S. Pat. No. 6,136,577 describes a method and apparatus for converting waste gases into ethanol using Clostridium ljungdahlii ATCC No. 55988 and 55989.
Acetogenic bacteria require a constant feed of nitrogen in the form of ammonia for stable performance and ethanol productivity. Most typically, the ammonia source is ammonium chloride provided in a low pH medium stream. The use of ammonium hydroxide is preferable due to cost and availability. However, because ammonium hydroxide is a base, it must be added as a separate medium stream. This addition of a high pH stream has the potential of causing fermentation operational issues. In addition, at higher productivity levels (>50STY) during the use of a more concentrated medium, ionic strength of the fermentation broth increases to a level that causes detrimental effects on culture performance.